1. Field of the Invention
This invention relates to liquid crystal light valves which modulate readout optical beam in response to an input electrical beam pattern.
2. Description of the Prior Art
Liquid crystal light valves (LCLVs) generally employ twisted nematic liquid crystal cells in which the liquid crystal molecules are ordered such that plane-polarized light passing through the cell is rotated 90.degree.. When sufficient voltages are applied to particular areas of the cell, the molecular axes of the liquid crystal molecules in these areas orient themselves parallel to the applied electric field. Polarized light can then pass through these areas of the cell essentially unchanged, while light is blocked in the areas of the liquid crystal to which no field is applied. Optical images can thus be produced by the spatial voltage pattern applied to the device.
Past implementations of LCLVs have included devices that are laser addressed, and devices that are photoconductor coupled to a cathode ray tube (CRT) or other light source. Laser-addressed systems, while exhibiting high resolution, are storage devices that may require several seconds to fill the screen, and use generally complex optical systems for both addressing and projection. Photoconductor-coupled devices, in addition to the addressing optics, have used either ultilayer construction to isolate the projection and addressing light, or have severe constraints on the wavelength of the addressing/projection light.
An electron-beam addressed LCLV could, at least conceptually, eliminate prior problems associated with photoconductors and light blocking layers. In addition, such a device could be made sufficiently fast for realtime operation. The most current electron-beam addressed LCLV prior to the present invention is a Tektronix Corp. device described in an article by Duane A. Haven, "Electron-Beam Addressed Liquid-Crystal Light Valve", IEEE Transactions on Electron Devices, Vol. ED-30, No. 5, May 1983, pages 489-492. This article describes an LCLV device in which electrons form a writing gun impinge upon a target electrode, and are subsequently erased from the target by means of flood guns. The flood guns recharge the target electrode to a controlled uniform potential. One problem associated with this device is that flood guns are not uniform, and produce a non-uniform distribution of electrons on the target electrode. This in turn produces an image that is not uniform. Image retention is another problem. The low energy flood gun is not capable of totally erasing the image created by the deep implant of electrons by the high energy writing gun. The Tektronix device requires the use of an expensive transmissive CRT, and its optically transmissive nature limits the choice of materials that can be used in connection with the LCLV. Some materials which might otherwise be suitable are birefringent and can rotate the polarization of the readout light, and therefore are not suitable. It would also be desirable to increase the sensitivity of the device.